1. Field of the Invention
The present invention relates to a method for the continuous creation of a bainite structure in a carbon steel, particularly in a strip steel, in accordance with the preamble of claim 1, and to a corresponding device in accordance with claim 24.
Tempering of carbon steel, particularly of strip steel, using a pass-through method, is a method for influencing the strength properties of strip steel that is frequently practiced. In this connection, the strip steel is first hardened, by means of heating with subsequent cooling, in corresponding pass-through devices, and afterwards changed with regard to its impact resistance, by means of annealing followed by cooling. In this connection, the heat required for tempering both during hardening and during annealing can be introduced into the strip steel in different ways, for example by means of inductive heating, conductive methods, or also by passing the strip steel through hot baths or gas flames. In this connection, how the heat is introduced into the strip steel and how it is conducted out of the strip steel again during cooling has a significant influence on the material properties of the strip steel that are adjusted. Very many different possibilities for influencing the material properties of the strip steel result from the structure conversions and the speed of the heating and cooling processes, respectively, and the holding times at established temperatures and structures that are placed between them. Frequently, carbon steels are used as the material for such strip steel.
A particularly preferred structure in the tempering of carbon steels is the so-called bainite. The bainite structure has very good properties with regard to the processability of work pieces having such a bainite structure, and is therefore required in a great number of industrial applications, particularly also as a relatively thin sheet-metal material or the like. In this connection, the production of such a bainite structure must be followed very closely with regard to the treatment temperature, in order to bring about the desired structure not just as an approach, but rather in the entire structural structure of such a carbon steel. Thus, the production of a pure bainite structure, in particular, is a metallurgically complicated and error-prone procedure. It is of particular importance, in this connection, that after the carbon steel is austenitized, the creation of the bainite structure is guaranteed with precise adherence to isothermal conditions, so that no other structure components that impair the bainite structure are maintained in the carbon steel. A simple purpose of use, which makes few demands on the quality of the bainite structure, is the production of packaging strips.
2. The Prior Art
Depending on the quality of the bainite that is required, different production devices and therefore also process managements are fundamentally known for the production of different bainite structures. Thus, it is known from U.S. Pat. No. 6,632,301 B2, for example, to pass a strip steel through a metal melt bath for quenching after austenitizing, and to clean the strip of residues of the metal bath, at least for the most part, after the strip steel has been passed through the metal bath, whereupon the strip is passed and deflected in a chamber, in meander form, in which chamber the isothermal conversion of the carbon steel to the bainite structure takes place. A disadvantage of this method of procedure is that the properties of the carbon steel with regard to bainite formation are worsened by means of the many windings of the carbon steel in the isothermal conversion unit, and also, the planarity of such strips decreases.
Other methods for the production of a bainite structure are known, which bring about a conversion of the carbon steel entirely in a metal bath or the like, which means that the pass-through length and thus the metal bath itself must have very great dimensions, and therefore high fixed costs for filling the bath with metal or salt, and problems in the temperature expansion of this salt bath are brought about.